1. Field of the Invention
This invention relates to the desulfurization of steel being vacuum treated in a vacuum degassing unit such as an RH degasser, with use of a desulfurization agent comprising calcium oxide, calcium fluoride and magnesium oxide in lump form added to the steel in the vacuum chamber of the degasser.
2. Description of Related Art
U.S. Pat. No. 4,661,151 discloses a first treating agent for steel desulfurization comprising CaO, CaF.sub.2, and from 10 to 60 weight % MgO, which agent, in fine powder form, is injected, with the aid of an inert carrier gas, into a ladle of molten steel and below a passage extending downwardly through a slag layer on the metal in the ladle and upwardly leading to an inert atmosphere treating chamber or a reduced pressure treating chamber such as an RH degassing apparatus. The desulfurization agent has a weight ratio of (%CaF.sub.2)/[(%CaO)+(%CaF.sub.2)].times.100%=20 to 80%. The inert gas and entrained fine desulfurizer particles float upward through the passage and into the treating chamber. The CaO and CaF.sub.2 serve as desulfurizers, and the MgO serves to protect the refractories in the equipment from erosion. A second desulfurization treating agent, comprising CaO and CaF.sub.2, also is used.
In U.S. Pat. No. 4,517,015 there is disclosed a steel refining method comprising blowing an inert gas into the steel from below and from above the surface of a slag overlying the molten steel. The inert gas can carry a desulfurization agent.
U.S. Pat. No. 4,341,554 discloses a steel desulfurization process in which molten metal in a ladle is covered with a synthetic slag, particulate lime, of a size retained on a number 80 sieve, is added to cover the slag, then finely powdered lime is injected below the slag surface and rises in the bath to combine with the larger lime particles to form a crust which deters entry of air into the bath.
In U.S. Pat. No. 4,277,279, there is disclosed a method and apparatus for dispensing a fluidized stream of particulate material useful for desulfurization of molten ferrous metals.
German patent application 254,216 A1 shows and describes a method of desulfurizing molten metal in an RH type treatment apparatus, wherein a solid calcium-containing desulfurizing agent, in filled wire form, is injected, without a carrier gas, into one leg of the RH vessel after the melt has been vacuum deoxidized and before it has been vacuum dehydrogenated. Sulfur thereby is reduced to below 0.005%, with Hz below 3 ppm.
Japanese published application No. 1129925 discloses addition, to molten steel in an RH vessel, a treating agent comprising Fe-Ca-Ni-Si, Ni-Ca, Ni-Fe-Ca or Cu-Ca alloy, or briquetted Fe-Ca, and coated with refractory and/or iron powder particles, in order to improve yield.
Japanese patent no. 82-112262 (WPI Access No. 84-039724/07) discloses use of an agent for desulfurizing stainless steel in a ladle, wherein the agent, comprising Cao-Al.sub.2 O.sub.3 -CaF.sub.2 and in amount of 13-16 kg/ton of steel, is added to the ladle before the steel melt is treated in an RH apparatus.
Japanese patent no. 1301814 discloses a method of refining steel by adding CaO to a ladle, filling the ladle and adding aluminum to the slag while blowing with Ar, then treating the steel in a vacuum degassing vessel with added MgO to solidify the slag.
Japanese 57067111 and 88-018646 (priority Jp 80-142220) discloses use of a calcium oxide-silica slag in a vacuum degassing vessel to control unelongatable non-metallic inclusions in a high-carbon steel to a low level.
Japanese 78017523 and 48009971 (priority JP 71-42943) discloses placing calcined lime on the bottom of a vacuum degassing vessel before introducing molten steel therein. The lime is sinter-bonded to the vessel bottom by residual heat of the vessel and does not float to the surface of the molten metal during vacuum treatment.